That depends. Please see apple support document HT207266 "About the compatibility of the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter" and also look at the specifications of your TB3 external hard drive.

The main point is that the TB3 external hard drive must be self-powered, not bus-powered; i.e. it must have a separate power cable/power supply, not be powered by the Thunderbolt connection.

Other than that, yes, it should work, though the TB3 drive will function at TB2's 20Gbps not 40Gbps.